The present invention generally relates to a liquid crystal display device. More specifically, the present invention is directed to a size design of a pattern formed on inner surfaces of substrates located opposite to each other in the liquid crystal device.
For a better understanding of the background of the present invention, an example of the conventional liquid crystal display device will now be described with reference to FIG. 1. FIG. 1 is a perspective view of this conventional liquid crystal display device. As illustrated in FIG. 1, a matrix pixel array is formed on an inner surface of one substrate 101. Each of pixel arrays is constructed of a pixel electrode 102 obtained by patterning a transparent conductive film and a thin-film transistor (abbreviated as "TFT" hereinafter) 103 used to drive the liquid crystal display device. A drain electrode of each TFT 103 is connected to the pixel electrode 102 located opposite to this drain electrode, a source electrode thereof is connected to a data line 104, and a gate electrode is connected to a scanning line 105. Such a substrate 101, which has these thin-film transistors (TFTs) formed in an integral form, will be hereinafter referred to as a "TFT substrate". In case that, for instance, polysilicon is employed as the semiconductor thin-film material of the TFT, since the high temperature process at approximately 1,000.degree. C. is carried out in the semiconductor manufacturing process, the TFT substrate 101 is made of a material having a better heat resisting characteristic, e.g., quartz. A color filter (CF) 107 made of three primary color (RGB) segments and a counter electrode 108 are stacked on an inner surface of the other substrate 106. The respective color filter segments are aligned with the pixels. A black mask 109 is patterned so as to shield the non-effective portion of the respective pixels and improve contrast of the liquid crystal display device. As described above, specifically, since the high temperature process is not carried out during the manufacturing process for the substrate 106 on which the color filter and the black mask and the like have been patterned (will be hereinafter referred to as to a "CF substrate"), relatively low cost materials such as glass may be used. A space defined between the TFT substrate 101 and the CF substrate 106 is filled with a liquid crystal layer 110. Furthermore, polarizing plates 111 and 112 are attached to outer surfaces of both of the substrates 101 and 106. When the TFT 103 is brought into the conductive state via the scanning line 105 for each row, an image (picture) signal supplied from the data line 104 is written into the respective pixel electrodes 102. In response to the written image signal, a voltage is applied between the pixel electrode 102 and the counter electrode 108 to change the molecular orientation or configuration of the liquid crystal layer 110. This change is derived as a variation of transmittance via one pair of polarizing plates 111 and 112 to display an image. The liquid crystal display device with the above-explained structure is called an active matrix type liquid crystal display device. It should be understood that the liquid crystal display device (LCD) in accordance with the present invention is not limited to this active matrix type LCD, but also any other type of LCDs.
Next, a brief description will be made of the assembling stage of the liquid crystal display device shown in FIG. 1 with reference to FIGS. 2A-2D. First, the TFT substrate 101 and the CF substrate 106 are prepared at a first stage shown in FIG. 2A. As previously stated, this TFT substrate 101 is made of quartz or the like having the heat resisting characteristics, and the thin-film transistors and the pixel electrodes are formed on one surface of the TFT substrate 101 is an integral form. It should be noted that the thermal expansion coefficient is relatively small, on the order of 5.times.10.sup.-7 cm/.degree.C. On the other hand, the CF substrate 106 is made of glass and the like, and the color filters, black mask, counter electrodes and the like have been previously formed on one surface of this CF substrate 106. It should also be noted that the thermal expansion coefficient is relatively large, on the order of 40.times.10.sup.-7 cm/.degree.C. As a consequence, the thermal expansion coefficient of the TFT substrate 101 is greatly different from that of the CF substrate 106 by 1 order. It should be understood that the orientation process has been previously performed on one surface of the respective substrates. Thereafter, a sealing member 113 is printed and applied along the periperal portion of the TFT substrate 101 as shown in FIG. 2B. Alternatively, the sealing member 113 may be printed and applied to the CF substrate 106 instead of the TFT substrate 101. This sealing member is made of thermosetting adhesive agent. Both of these substrates 101 and 106 are positionally aligned with each other at the subsequent stage (see FIG. 2C). That is, the pixel electrode pattern formed on the TFT substrate 101 is mutually and positionally aligned with the black mask pattern formed on the CF substrate 106. Finally, a heating process at approximately 150.degree. C. is carried out with giving constant weight to harden the sealing member 113 made of the thermosetting adhesive agent at a stage shown in FIG. 2D. As a result, the TFT substrate 101 and the CF substrate 106 are attached to each other with a predetermined space therebetween, so that a so-called "cell structure" is obtained. Finally, a liquid crystal layer is sealed and filled within this predetermined space, whereby the liquid crystal display device is accomplished. It should be noted that although the thermosetting adhesive agent is employed as the sealing member in the above-described example, ultraviolet hardening type adhesive agent may be alternatively utilized as disclosed in, for instance, Japanese Patent KOKAI (Laid-open) Application No. 61-112128 (1986). However, it is practically difficult to obtain sufficient adhesive strength by the ultraviolet hardening type adhesive agent, which may cause poor reliability. As a result, even when such a ultraviolet hardening type adhesive agent is employed, the heating process (for example, at approximately 90.degree. C.) is carried out at the succeeding stage to emphasize adhesive strengths.
FIG. 3 is a sectional view of an active matrix type liquid crystal display device from which one pixel portion is cut away. The pixel is formed on the TFT substrate 101. This pixel is constructed of a pixel electrode 102 and a thin-film transistor 103 for driving this pixel electrode 102, and the like. In addition, a data line 104 used to supply an image (picture) signal to the thin-film transistor 103, and also a scanning line 105 used to supply a selecting signal are fabricated. The CF substrate 106 is arranged at a predetermined space with respect to this TFT substrate 101. A counter electrode 108, a black mask 109, and a color filter 107 are formed on an inner surface of the CF substrate 106. A liquid crystal layer 110 with, for example, twisted nematic orientation is held between the TFT substrate 101 and the CF substrate 106. An opening 114 aligned to the pixel electrode 102 is formed in the black mask 109. That is, an effective display region on the pixel electrode 102 is exposed and also a non-effective display region is shielded from incident light by way of the black mask 109. This non-effective display region includes the above-described thin-film transistor 103, data line 104, and scanning line 105 and so on.
As previously explained, the black mask 109 is used so as to improve display contrast by irradiating the incoming light only to the effective display region. As a consequence, the pattern of the pixel electrode 102 must be precisely aligned with the pattern of the black mask 109. Assuming now that the pattern of the pixel electrode 102 would be positionally shifted from the pattern of the black mask 109, a so-called "light pass-through" phenomenon happens to occur in the non-effective display region by the opening 114 of the black mask 109, resulting in lowering of display contrast. In general, approximately 1 micron is required for the positional alignment precision of these patterns in case of such a compact display device having a size of approximately 1 inch when high precision could be achieved. However, there is a great difference of 1 order between the thermal expansion coefficients of the TFT substrate 101 and the CF substrate 106. Even when the positional alignment between these substrates 101 and 106 would be performed before assembling the cell, a positional shift would be produced after the liquid crystal display device has been assembled by performing the heating process. The positional shift error would become about 2 microns, for instance. Accordingly, if no measure is taken, then such a problem as the so-called "light pass-through" phenomenon would be produced, resulting in deterioration of the display contrast.